This invention relates to inductive power transfer; more particularly to loosely coupled systems for inductive power transfer, and in particular this invention relates to protection means for limiting the amount of current circulating in a secondary pickup coil of an inductive power transfer system.
The general structure of an inductive power transfer installation is that there is a primary conductor (or more) energised with alternating current, and one or more secondary or pickup devices which intercept the changing flux surrounding the primary conductor and convert this flux into electrical energy by means of windings. Often the pickup devices are mobile, and move alongside, or sometimes (if internal energy storage is available) away from the primary conductors.
There appears to be at least two distinct groups of inductive power transfer systems amongst the published literature. One group uses a xe2x80x9cspread-out transformerxe2x80x9d approach for the primary trackway, in which a series of iron laminations is used along the full length of the trackway to enhance coupling of the flux to an adjacent set of laminations comprising the flux concentrating means used to cause the collected flux to traverse the (sometimes) resonant pickup windings. The energising frequency is relatively low (from mains frequency up to about 5 kHz). Often the primary trackway is buried in a road and faces upwards to link with pickups beneath a road vehicle that face downwards. This approach provides tight coupling, and power is received essentially as if it arrives from a constant-voltage source. Examples of this type of approach are illustrated in a series of patent specifications from Bolger (e.g. U.S. Pat. No. 4007817 or FIG. 1 of U.S. Pat. No. 4800328). Klontz et al ( U.S. Pat. No. 5157319) describes an alternative tight coupling, involving a coaxial winding transformer secondary encircling a primary conductor.
Our group uses as the primary trackway an elongated loop formed from a parallel pair of conductors, without ferri/ferromagnetic material, and flux is coupled through the core (which does include ferri/ferromagnetic material) to the windings of the resonant pickup coil. This coupling is described as loose. Some versions of the track are provided with lumped resonance elements. The delivery of power is controlled by decoupling at the pickup, using a number of disclosed techniques, and because the system uses a resonant circuit as part of the pickup, the current At produces appears to come from a constant-current source. The energising frequency is relatively high (10-30 kHz), and in some examples the primary trackway is mounted upon a conveyer rail, facing sideways to couple with pickups upon self-powered conveyer units, although it is in other instances embedded within a roadway. This type of approach is illustrated in a series of patent specifications from Boys and Green, commencing with WO 92/17929.
For a comparison of these two approaches refer to FIGS. 14 and 15, and also a conventional transformer (FIGS. 12 and 13), in terms of transformer equivalent circuits. (All FIGS. 12-17 are prior art). FIG. 12 shows an ordinary, tightly coupled transformer having a primary winding 1201 and a secondary winding 1202. FIG. 13 shows the transformer equivalent circuit, where winding 1301 represents the coupled flux M, winding 1302 represents the leakage flux about the primary, and 1303 represents the leakage flux about the secondary. The value of M is obtained from M=k{square root over (L)}1L2 where k is typically 95% or more. FIG. 14 shows a loosely coupled inductive power transfer pickup, with primary conductors 103 and 104, a core 300, and a resonant circuit comprising inductor 1401 and capacitor 1402. Considering FIG. 15, the equivalent inductance 1504 (M) represents the power coupling (shared) component of the flux while 1503 is the leakage flux (such as the flux radiated from the core of the pickup while it is carrying a significant resonating current). For loosely coupled systems having a primary pathway in air, the ratio of inductors 1503:1504 is typically 0.7:0.3 whereas for the iron-cored primary with iron-cored secondary devices of Bolger and others the ratio is typically more like 0.2:0.8. In FIGS. 15 and 17, the trackway (constant-current source 1500 with equivalent inductors 1501, 1502) supplies a constant current. FIG. 16 shows a kind of inductive-power transfer device where winding 1601 is a resonant, controllable winding and 1602 with rectifier 1605 supplies useful powerxe2x80x94such as a constant current source for battery charging. Considering the transformer equivalent circuit of FIG. 15, the value of the short circuit current (if the output was to be shorted) is       I    sc    =      IM                  L        1503            +      M      
where M is the inductance of 1504.
We have devised a battery charger which employs loosely coupled inductive power transfer, the subject of patent application PCT/NZ97/00053. Considering a practical circuit from the battery charger in transformer equivalent form, as shown in FIGS. 16 and 17; items 1705 and 1706 represent leakage inductance from the actual windings; 1705 for the large number of turns in the resonating/control winding, and 1706 for the power collection winding. The relative proportions of L in FIG. 17 are: 1504=30%, 1503=65%, 1705=c. 5%., and 1706=c. 5impedance of the primary section as seen looking back from the inductors 1503, 1504 (=95% of L) can then be derived by assuming that a short circuit is placed at 1708 (dashed line) and is   Z  =                              (                                    5              ⁢              %              ⁢              L                        +                          X              c                                )                ⁢        95        ⁢        %        ⁢        L                              5          ⁢          %          ⁢          L                +                  X                      c            res                          +                  95          ⁢          %          ⁢          L                +                  5          ⁢          %          ⁢          L                      =    ∞  
Since the denominator at resonance is zero then Z is infinite; thereby providing the basis for stating that the source acts as a current source. A Bolger type circuit is equivalent to FIG. 13. The no-load voltage will be determined by the output impedance Z=L1303+L1302 when driven from a voltage source as is done in Bolger type circuits. The output impedance is Z=L1303+L1301 if the circuit were to be driven from a current source.
While the constant-current characteristic of this type of inductive power transfer system is generally an advantage, it does impose a risk should a pickup coil enter a state in which there is no control over the amount of current collected. A perfect constant-current source will have no voltage limit. An uncontrolled current resonating in a resonant secondary circuit forming part of a loosely coupled inductive power transfer system may build up to reach high levels if the circuit Q is large, whereupon a number of adverse results may occur, such as component failure, for example by overheating or breakdown of semiconductors or of dielectrics within resonating capacitors and apart from loss of function this can lead to the development of fire within the pickup device. Our usual methods for controlling secondary current rely on active control apparatus, actively causing a switching action about the resonant secondary when an over-voltage condition is detected by a voltage comparing circuit. Passive limiting, relying perhaps on the inherent bulk properties of materials should be safer than active control means. Reliance on active control can break down when several factors impinge together on a device so that active control becomes least likely to function when it is most needed. Some systems using loosely coupled (i. e. constant-current) inductive power transfer have been employed in situations where extreme reliability is a desired feature. If such systems rely solely on active control to restrict the circulating current, then in the absence of function by the active control it is likely that a catastrophic breakdown will occur.
Bolger and Ng in U.S. Pat. No. 4,800,328 (Jan. 24 1989) described the application of constant-voltage transformer principles to an inductive power transfer device by providing a saturable pickup core. This is a control application. The laminated iron core is intentionally provided with a saturable site of reduced cross-sectional area. During normal operating conditions the core is always saturated to a variable extent and the output from the pickup is limited accordingly by the amount of flux remaining within the core. Furthermore, the resonant frequency is designed to be less than that of the supply voltage at low loads, so that as the core moves into saturation, the resonant frequency rises towards the system frequency; coupling improves and more output (resembling a constant voltage) is available. Cores of this type, driven into saturation will evolve heat from hysteresis losses, and cooling is not provided for in the region of the constriction, so this approach would result in a quite temperature-sensitive output voltage. The inventors have consistently aimed for a constant-voltage approach.
Loosely coupled in relation to the transfer of inductive power means that the proportion of flux actually coupling the primary to the secondary is significantly less than the total magnetic flux present in the region of the coupling structures.
Ferrimagnetic properties occur in ferrite materials, in which the entire ferrite molecule contributes to the magnetic properties. In the main these are comparable to ferromagnetic properties; permeability, saturation, hysteresis, etc. occur in ferrimagnetic materials.
Ferromagnetic properties occur in iron, nickel, cobalt, gadolinium, and dysprosium, and their alloys, in which the magnetic properties reside in the atoms. Useful ferromagnetic materials for this application include powdered iron, sintered iron, amorphous iron wires, laminations of iron, silicon steel, grain-oriented steel; used alone or in combination.
Saturation is a property of ferri/ferromagnetic materials defined as a change in the permeability of the material as a function of the magnetic field, in which the material exhibits a finite capacity to carry a quantity of flux, so that the permeability falls as the field rises. An analogy to saturation is the way that a bath towel can absorb only a limited amount of water, after which the surplus water drips off.
It is an object of this invention to provide an apparatus or a method for controlling an inductive power transfer pickup, or at least to provide the public with a useful choice.
In a first broad aspect the invention provides apparatus for controlling an inductive power transfer pickup for use in a loosely coupled inductive power transfer system, said pickup being capable in use of collecting power in the form of a current source from a magnetic flux surrounding a primary conductor when energised with alternating current at a system frequency, wherein the pickup includes active control means capable of controlling an output voltage or output current, and wherein the pickup is a resonant circuit which is resonant at the system frequency, the pickup includes passive means capable of limiting the amount of a resonating current circulating in said pickup at less than a predetermined maximum amount, said passive means comprising at least one saturable inductor having a core; at least a portion of the core being capable of becoming saturated at a predetermined flux density; the saturable inductor being connected so as to carry at least a portion of the resonating current so that the onset of saturation within the core reduces the effectiveness of the collection of power and so causes the amount of the current entering the resonant circuit to be limited.
In a related aspect the invention provides apparatus as previously described wherein the core capable of becoming saturated is comprised of a ferrimagnetic material.
In a related aspect the invention provides apparatus as previously described wherein the core capable of becoming saturated is comprised of a ferromagnetic material.
In a related aspect the invention provides apparatus as previously described wherein the at least one saturable inductor is constructed so that the saturable portion of the core is shared by both a coupling flux and by a leakage flux, so that the onset of saturation causes the amount of coupling flux to be diminished and hence the amount of current entering the resonant circuit from the current source is also diminished and so that the onset of saturation results in a minimal amount of detuning.
In another related aspect the invention provides that at least one saturable inductor is selected to exhibit an onset of saturation within the core at or above a selected current so that the onset of saturation within the core changes the resonant frequency of the pickup and so causes the tuning of the pickup to deviate from the system frequency, thereby reducing the effectiveness of the collection of power and so causing the amount of the current entering the resonant circuit to be reduced.
In another related aspect the invention provides that at least one saturable inductor is constructed so that, when in use, the saturable portion of the core is shared by both a coupling flux and by a leakage flux, so that the onset of saturation causes the amount of coupling flux to be diminished and hence the amount of current entering the resonant circuit from the current source is also diminished and so that the onset of saturation results in a minimal amount of detuning.
In a further related aspect the invention includes a core capable of intercepting the flux; the core having a saturable part having a restricted cross-sectional area capable of exhibiting an onset of saturation at a predetermined flux density so that the efficiency of coupling between the primary conductor and the pickup circuit is reduced if the material becomes at least partially saturated.
In a yet further related aspect the invention provides that the predetermined flux density at which the onset of saturation may occur is determined by selecting a material having known saturation threshold properties from a range of ferrimagnetic or ferromagnetic materials and using an amount of the selected material within a flux-carrying part of the core so that the efficiency of coupling between the primary conductor and the pickup circuit is reduced if the material becomes at least partially saturated.
In a related aspect the invention includes a procedure in which the amount of flux required to reach an onset of saturation is controlled by passing current through one or more additional windings wound over a portion of the core having a predetermined onset of saturation; the windings being capable of carrying a DC current capable of generating a flux within the saturable portion of the core; the DC current being generated by a controller responsive to power pickup conditions during use, so that the efficiency of coupling between the primary conductor and the pickup circuit is thereby controllable.
In a yet further related aspect the invention provides that the saturable inductor is physically separate from an inductor capable of intercepting the magnetic flux, and the saturable inductor is connected within the resonant circuit so that the saturable inductor carries at least a proportion of the total resonating current, and so that the onset of at least partial saturation in the saturable inductor during use causes the resonant frequency of the pickup to move away from the system frequency.
In a second broad aspect the invention provides a method for operating a resonant inductive power pickup device for an inductive power transfer system wherein the magnitude of a circulating resonant current within the pickup device is capable of being limited so as to remain below an intended magnitude as a result of at least partial saturation being reached within a saturable core of an inductor included within the resonant circuit of the device, the limiting process being independent of an active control means, so that a voltage limit is provided.
In a third broad aspect the invention provides a method for operating a resonant inductive power pickup device for an inductive power transfer system, wherein the magnitude of the circulating resonant current within the pickup device is controllable as a result of saturation being caused within a saturable inductor included within the resonant circuit of the device by a magnetising current passed through at least one additional winding; the magnetising current being provided by an active control means.
In a fourth broad aspect the invention provides apparatus for controlling the amount of power within a power pickup device having a secondary pickup inductor, having a ferromagnetic core, included in a resonant circuit, wherein the apparatus employs a physical property (apart from a permeability greater than that of air at normal temperature for non-saturating amounts of magnetic flux) of the core, wherein the physical property is deliberately predetermined so that the core behaves in a manner capable of limiting the pickup of power when operating under conditions outside normal use of the pickup.
In a fifth broad aspect the invention provides apparatus for controlling the amount of power within a power pickup device as described previously, wherein the apparatus includes a ferromagnetic core including in its magnetic circuit at least a portion of material selected to exhibit a preferably reversible reduction in permeability with a rise in temperature; the permeability reaching substantially 1.0 at the Curie point, so that in the event of the core reaching too high a temperature the permeability of the core is reduced, thereby limiting the voltage circulating within the resonant circuit.
In a sixth broad aspect the invention provides a method for operating an inductive power pickup device for an inductive power transfer system wherein the output of the pickup device is controlled as a result of saturation being reached during normal use in a ferromagnetic pickup core within the device.
In a related aspect the invention provides a method for operating an inductive power pickup device for an inductive power transfer system wherein potentially catastrophic circulating resonant currents within the pickup device are limited either as a result of saturation being reached within a ferromagnetic pickup core within the device, so that the inductance of the inductor is altered and the amount of power transferred is reduced.